The present invention relates to optical communications. More particularly, the present invention relates to a tunable filter for use in conjunction with optical communications systems.
FIG. 1a depicts a simplified schematic diagram of atypical WDM network 100 in the prior art. WDM network 100 includes a plurality of transmitters TX-1 through TX-n. Each of the transmitters includes an optical source for generating an optical signal xcex-i, i=1, n. Each optical signal xcex-i is characterized by a unique peak wavelength onto which information may be modulated in well-known fashion. The plurality of optical signals xcex-1 through xcex-n are combined into a single xe2x80x9cmultiplexedxe2x80x9d signal m-xcex by wavelength multiplexer 102, and the multiplexed signal m-xcex is then launched into optical fiber 104.
A plurality of subscriber terminals (e.g., 106-S1, 106-S2 and 108-S1 through 108-Sn) are in optical communication with network 100. Each such subscriber terminal includes a receiver(s) (not shown) for receiving information that is carried over network 100 via multiplexed signal m-xcex. An individual subscriber terminal may subscribe to the information contained on only a single channel (i.e., on a single optical signal xcex-i) of multiplexed signal m-xcex.
Subscriber terminals 108-S1 through 108-Sn located at end terminal 108 require, collectively, most or all of the individual channels xcex-1 through xcex-n multiplexed signal m-xcex. To provide such channels to subscriber terminals 108-S1 through 108-Sn, multiplexed signal m-xcex is typically demultiplexed, fully resolving it into its constituent channels. Demultiplexer 110 is used for that purpose.
Subscriber terminals 106-S1 and 106-S2 are located at xe2x80x9csmallxe2x80x9d intermediate node 106. Node 106 requires only a few of the channels of multiplexed signal m-xcex (ie., terminal 106-S1 receives only channel xcex-1 and terminal 106-S2 receives only channel xcex-3). As a consequence, rather than fully demultiplexing multiplexed signal m-xcex at node 106, only the required channels are dropped (i.e., removed or separated) from multiplexed signal m-xcex and delivered to the appropriate subscriber terminal. One or more xe2x80x9cwavelength xe2x80x9c(add)/dropxe2x80x9d filters (i.e., filters 106-WAD1, 106-WAD2), which are operable to drop a single channel, are advantageously used for this purpose.
For example, in network 100 at node 106, add-drop filter 106-WAD1 separates and drops channel xcex-1 from multiplexed signal m-xcex. Channel xcex-1 is then transmitted to subscriber terminal 106-S1. Also, add-drop filter 106-WAD2 separates and drops channel xcex-3, which is then transmitted to subscriber terminal 106-S2. As the name implies, in at least some embodiments, wavelength add-drop filters are operable to add a single channel having the same characteristic wavelength as the drop channel. For example, in network 100, transmitter 106-T1 generates signal xcex-1 that is added to multiplexed signal m-xcex via 106-WAD1. Alternatively, such a channel may be added to the multiplexed signal elsewhere in network 100.
It will be clear to those skilled in the art that a typical WDM optical communications network will have many more nodes and typically includes many other elements (e.g., amplifiers for maintaining signal strength, etc.) than are depicted in FIG. 1a. These other nodes and other elements are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention.
FIG. 1b depicts a known wavelength add-drop filter. The particular filter depicted in FIG. 1b is a Fabry-Perot etalon filter, well known in the art. Etalon filter 150 consists of a pair of highly reflective dielectric mirrors M1 and M2 that are separated by a precisely defined gap G. An optical cavity OC is defined between opposed surfaces SM1 and SM2 of the final dielectric layer of each mirror.
A multiple-wavelength signal MWS-IN from input waveguide (e.g., an optical fiber) F-IN is collimated by lens L1 and illuminates the mirrors M1 and M2. Most of wavelengths of signal MWS-IN are reflected from the filter and couple into output waveguide F-OUT. Signals Dxcex1-Dxcexj having a wavelength within a very narrow range or xe2x80x9cpassbandxe2x80x9d are, however, transmitted through the mirrors, pass through lens L2, and couple into drop waveguide F-D. Any signals Axcex having a wavelength within the narrow pass band of the filter can be delivered to filter 150 from xe2x80x9caddxe2x80x9d waveguide F-A and coupled into output waveguide F-OUT.
Performance parameters of the etalon filter 150, such as reflectivity/transmissibility, passband, center transmission wavelength of the passband and finesse are readily calculable and are dependent on properties of the optical cavity OC (i.e., gap G) and mirror reflectivity and the coupling efficiency into output waveguides.
Returning to illustrative network 100, to xe2x80x9cdropxe2x80x9d two channels (e.g., xcex-1 and xcex-3) from multiplexed signal m-xcex, two add-drop switches (e.g., implemented as described above) can be used. Alternatively, it is possible to drop the same two channels using a single xe2x80x9ctunablexe2x80x9d etalon filter having an adjustable passband xe2x80x9ccenterxe2x80x9d wavelength. The xe2x80x9ccenterxe2x80x9d wavelength is the predominant wavelength of the passband (hereinafter xe2x80x9ccenter transmission wavelengthxe2x80x9d).
In such tunable etalon filters, one of the two mirrors is typically placed on a translation actuator (e.g., a piezoelectric transducer) that is under electrical control. Moving the actuator changes the size of the gap between the mirrors. Since the gap (size) controls the center transmission wavelength of the filter, moving the actuator changes that center transmission wavelength.
A problem exists, however, with existing tunable filters. As explained above, to change the center transmission wavelength, the size of the gap between the two mirrors is altered. In doing so, the gap will assume a number of intermediate sizes until the desired size is attained. At such intermediate gap sizes, the optical cavity will tune to channels or signals having intervening wavelengths (hereinafter xe2x80x9cintervening channelsxe2x80x9d or xe2x80x9cintervening signalsxe2x80x9d). Such intervening signals will be transmitted by the filter, delivered to the drop fiber and passed to the subscriber terminal rather than to the intended destination. To prevent intervening signals from being delivered to a subscriber terminal in this manner, those signals must be disadvantageously temporarily interrupted while tuning the filter to a new center transmission wavelength.
The art would therefore benefit from a tunable filter that, during tuning, does not disrupt intervening channels.
Some embodiments of the present invention provide a tunable filter without some of the disadvantages of the prior art. In particular, the illustrative embodiment of the present invention is a tunable filter that does not interrupt intervening channels during tuning.
In accordance with the illustrative embodiment of the present invention, a tunable filter includes an optical cavity, a tuning device and a filter-disabling device. The length of the optical cavity defines the center transmission wavelength of the filter. Other attributes of the optical cavity and the mirrors define the finesse of the filter.
As used herein, the term xe2x80x9cpassbandxe2x80x9d refers to the range of wavelengths that are transmitted or passed by a filter, the term xe2x80x9ccenter transmission wavelengthxe2x80x9d refers to the predominant or peak wavelength in the passband, and the term xe2x80x9cfinessexe2x80x9d refers to the transmissibility of the filter. The term xe2x80x9cfinessexe2x80x9d is also properly considered to be a measure of the xe2x80x9csharpnessxe2x80x9d of the transmission peak of the filter. And, as will be appreciated by those skilled in the art, the term xe2x80x9cfinessexe2x80x9d also has mathematical definitions (e.g., assuming equal reflectivity mirrors: finesse=4r/(1xe2x88x92r2), where xe2x80x9crxe2x80x9d is the reflectivity of the mirrors). In the context of the present invention, the term xe2x80x9cfinessexe2x80x9d is intended to refer to the transmissibility of the filter, as will become clear later in this Specification.
The tuning device is operable to change the center transmission wavelength of the tunable filter. The filter-disabling device is operable to temporarily disrupt the finesse or otherwise substantially lower the transmissibility of the optical cavity, thereby preventing the transmission of any wavelengths through the tunable filter. In some embodiments, filter transmissibility is lowered by disrupting the reflectivity (along the optical axis) of at least one of the two mirrors defining an optical cavity.
In accordance with the present teachings, before changing the center transmission wavelength, the filter-disabling device is enabled. Doing so disrupts the finesse of the optical cavity thereby substantially preventing the transmission of any optical signals through the filter. The tuning device is then used to change the center transmission wavelength. Even though the tuning device will tune to undesired intervening channels during the tuning process, the filter will not transmit such intervening channels since the finesse of the optical cavity is disrupted (i.e., the transmissibility of the filter is low). After tuning is complete, the filter-disabling device is defeated and the filter exhibits its desired transmission characteristic (i.e., transmits the desired channel through the filter).
The inventive concept may be implemented in a variety of ways. Several illustrative embodiments are summarized below and described in more detail later in this Specification.
In some embodiments, the optical cavity comprises two spaced-apart mirrors. In a few of those embodiments, one of the mirrors is movable and functions as the tuning device. In particular, moving the movable mirror changes the length of the optical cavity thereby changing the center transmission wavelength of the filter. The movable mirror can be readily implemented using well-known surface micromachining techniques (e.g., micro-electromechanical systems xe2x80x9cMEMSxe2x80x9d).
In one embodiment, filter-disabling device comprises an arrangement for tilting or rotating one of the mirrors. Tilting a mirror disrupts the finesse of the optical cavity such that the filter becomes reflective of all wavelengths of light.
In other embodiments, the optical cavity is modified wherein one of the two mirrors is xe2x80x9csplitxe2x80x9d into two groups of layers of dielectric material that are separated by a gap. Thus, the filter has two xe2x80x9cgaps,xe2x80x9d a primary gap between the first and the second mirror (tuning device), and an auxiliary gap (filter-disabling device) that divides the layers of the xe2x80x9csplitxe2x80x9d mirror. Both the primary gap and the auxiliary gap are variable. When the auxiliary gap is at a (readily) predetermined size, the finesse of the filter is at a maximum. By appropriately changing the auxiliary gap, the finesse of the filter is disrupted and the filtering function is defeated.
In other embodiments, the filter-disabling device comprises an electrically-switched absorbing, scattering or depolarizing media that is disposed in the optical cavity. By applying a voltage, the optical characteristic of the media can be changed from non-transmissible to transmissible (or visa-versa). The cavity is tuned (i.e., the center transmission wavelength is changed) by changing the length of the optical cavity.
In still other embodiments, an optical cavity incorporates a semiconductor optical amplifier that provides either a zero-loss or highly lossy roundtrip as a function of whether current is flowing through the optical amplifier.